In modern control systems for internal combustion engines, the power of the engine and especially the torque thereof is controlled via electric paths in dependence upon at least one input value. This input value can be a driver command derived from the position of an operator-controlled element actuable by the driver. An example of such a control system is disclosed in U.S. patent application Ser. No. 09/983,090, filed on Jan. 12, 1998, which corresponds to International patent publication WO-A 97/43531. There, a desired torque value of the engine is determined in dependence upon the driver command. The power variables of the engine such as air input, fuel input and ignition angle are controlled in dependence upon this desired torque value so that the actual torque of the engine corresponds to the desired torque. In the normal case, the target is to adjust the control variables with the objective of an optimal torque of the engine so that the engine can operate at high efficiency. This means that, for example, the ignition angle adjustment must be located always close to the optimal ignition angle.
In some operating states, for example, in idle, it can, however, be desirable to deteriorate the efficiency of the engine and to select the ignition angle shift so that the control of the torque is possible exclusively via the ignition angle in both directions, that is, to increase torque and to decrease torque. Since the total torque should not change, the air supply to the engine is correspondingly increased in this operating state. The extent of the deterioration of efficiency is determined by a so-called torque reserve value which is an index for the maximum possible change of the torque via the ignition angle adjustment in the sense of increasing the ignition angle.
With this measure, the dynamic and precision of the torque control is improved in the operating states mentioned there, namely, idle, start and for active catalytic converter heating.